1. Field of the Invention
This invention relates to an electrophotographic element and in particular an electrophotographic element utilizing an organic photoconductive material having excellent photosensitivity and stable characteristic properties.
2. Discussion of the Prior Art
Various photoconductive materials have been proposed including inorganic substances such as selenium and zinc oxide, organic low-molecular weight compounds such as anthracene, perillene, pyrazoline, and imidazole, and organic polymeric compounds such as polyvinylcarbazole and polyvinylanthracene. Organic photoconductive materials are generally advantageous in that (a) they are very transparent when formed into a film and in their film-forming property, (b) they exhibit favorable plasticity, and (c) they are available at low cost. Accordingly, many attempts have been made to utilize the organic photoconductive materials in practical applications. However, organic photoconductive materials have important drawbacks in that (a) they are considerably lower in photosensitivity when compared with inorganic photoconductive materials such as selenium and zinc oxide and (b) the spectral sensitivities of organic photoconductive materials are disadvantageously biased toward the microwave range. Therefore, various efforts have been made to improve the sensitivities thereof. However, though a number of studies concerning organic photoconductive materials suitable for use in electrophotography have been made in recent years, there are few known organic photoconductive materials which have a high enough level of photosensitivity as to be utilizable in practical applications. For example, Japanese Patent Disclosure Publication No. 94337/1974 describes an organic photoconductive material which comprises a photoconductive or semiconductive organic compound and an electron acceptor composed of mononitro-, dinitro- or trinitrodibenzothiophenesulfoxide or dibenzothiophenedioxide, and which exhibits favorable properties in either negative or positive charging polarity. However, this organic photoconductive material is not considered satisfactory in practical applications due to its still low photosensitivity. Moreover, the above-mentioned patent publication described that the organic photoconductive material may further comprise (a) one or more photoconductive materials other than the first-mentioned photoconductive organic material and the mononitro-, dinitro or trinitrodibenzothiophenesulfoxide or dibenzothiophenedioxide, and/or (b) one or more photoconductive sensitizing agents. However, third or sensitizing additives which would have a substantial effect on the photosensitivity of the organic photoconductive material are not particularly mentioned in the specification of this patent publication. Further, even if there are added to the organic photoconductive material a photoconductive material other than the electron acceptor mentioned above and a sensitizer, the sensitization of the organic photoconductive material generally increases at most from several percent to several tens percent. In some cases, the sensitization may decrease by such addition. Also, it is considered difficult to enhance the sensitivity of an organic photoconductive material to a substantial extend by adding thereto a sensitizer and other photoconductive materials without impairing the important feature that the organic photoconductive material can be charged either positively or negatively.